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The stuff is kept behind several locked doors in an unmarked brick building on Rogers Street, where Joule researchers are still tinkering with how the organism, which no one here will name, consumes sunlight and carbon dioxide, then sweats ethanol. The 2-year-old company, which just made public news of its possibly revolutionary process, said that one day soon, its SolarFuel could be used to power vehicles. But until then, Joule officials said they intend to keep quiet about just what goes into their product.

On a recent visit, however, chief executive Bill Sims and cofounder David Berry did provide a peek at the Joule process, while dodging persistent inquiries about the mysterious fuel-making organism.

Despite its green color, Sims said, it is not algae. But he stops short of revealing more during a tour of the Joule plant.

Whatever it is, the stuff swirls inside a bottle being shaken by a machine called an agitator. When there’s enough of it, the gunk will go into a solar panel-like contraption - along with a mixture of brackish water and some nutrients - where it will use photosynthesis to produce ethanol, hydrocarbons, and petroleum-based chemicals.

“We are converting sunlight to liquid energy,’’ Sims explained. The goal is to make an environmentally-friendly fuel that can compete with $50-a-barrel fossil fuels.

Energy specialists at the Union of Concerned Scientists, which is headquartered in Cambridge, said such a process is possible, but without knowing exactly what Joule is working with, they can’t vouch for the company’s claims.

According to Jeremy Martin, a senior scientist in the clean vehicles program at the Union of Concerned Scientists, the organism is almost certainly a plant.

“All plants convert sunlight and CO{-2}, sugar, starch, or something, and we use that to make fuel,’’ Martin said. “It doesn’t sound like they’ve given us enough detail to know if this is an important breakthrough.’’

At the plant, researchers in white coats and safety glasses mill about a lab that resembles a high school chemistry classroom. Countertops are lined with petri dishes and bottles of unidentifiable substances.

Berry said Joule’s process, called helioculture, grew out of a desire to circumvent two fundamental challenges in the biofuel industry. The first is a dependency on large amounts of feedstock - such as grass, algae, or corn stover - that must be grown and harvested before fuel can be manufactured. The second is the difficulty most alternative fuel companies face when attempting to transition from making products in a small laboratory to manufacturing on a commercial scale.

“What we started out with in 2007 was the somewhat simplistic question of how do we eliminate these problems,’’ Berry said. Joule took inspiration from solar panels and came up with a similar device called a “solar converter.’’

The contraption is a clear, flat square that appears to have tunnels running through it.

“These are essentially the solar panels that drive the photosynthesis that leads to liquid energy rather than electric energy,’’ Sims said. The company estimates it will be able to generate 20,000 gallons of fuel a year for every acre of solar converter panels.

Some said that may be a tough benchmark to meet. The crucial question is not whether the Joule process works, but whether it is cost-effective.

“If they show you a device [like this solar converter], that is one square meter and captures the sun and puts out fuel, I mean, the basic thing to me is how much does that thing cost to build and operate and how much fuel do you get,’’ said Martin. “That’ll be the key to this thing: whether they can manufacture something that is so inexpensive it can work at scale.’’

Several other New England companies also are working on inexpensive alternative fuels.

One is Verenium Corp., a Cambridge firm making biofuel out of a type of sugar cane that is fermented into fuel using a basic form of the E. coli bacteria.

Russell Heissner, Verenium’s director of technology development, said his company is producing 1.4 million gallons of fuel a year at a demonstration facility in Louisiana. He welcomed competition from Joule.

“It’s a big, vast ocean of imported petroleum that we have to displace, so it’s not all going to be one solution,’’ Heissner said. “We know that there are competitors out there, but we like it.’’

Joule, which has fewer than 30 employees, has started testing its technology outside the laboratory. A pilot facility is in the works, Sims said, and could be operating by early next year.

For now, Joule researchers are testing how the designer organism fares in a number of environments and conditions. Despite the serious work, they seem to maintain a sense of humor about the cloak and dagger nature of the project.

“I’m not just mixing water. This is real science,’’ quipped research assistant John King, as he walked past with a bottle of clear fluid. “I’m a somebody.’’